This invention relates to facsimile transmitters and receivers of the type utilized to transmit and receive information-bearing signals representing the dark/light variations on a document located at the transmitter and converting the information-bearing signals to marks or images on a copy medium located at the receiver so as to form a copy which is a reasonable facsimile of the original document.
Such facsimile transmitters and receivers include scanning mechanisms which permit the document at the transmitter to be scanned for dark/light variations and a copy medium at the receiver to be scanned while reproducing the dark/light variations at the transmitter. In order to have a faithful facsimile, it is necessary to achieve a certain degree of synchronization between the transmitter and the receiver. In general, this requires that the rate of scanning motion at the transmitter scanning means equal the rate of scanning motion at the receiver scanning means. In addition, it requires that the position of the transmitter's scanning mechanism corresponds substantially to the position of the receiver scanning mechanism.
In order to achieve this type of synchronism, the scanning mechanism at the transmitter unit is, for a period of time, run at a different scanning rate than the scanning mechanism at the receiver. Typically, it is the receiver's scanning rate which is altered by first running the receiver's scanning mechanism at a rate at which differs substantially from the normal scanning rate which the transmitter is running. After a certain degree of synchronism is achieved, the scanning rate at the receiver is changed to substantially correspond with the scanning rate of the transmitter. In some instances, the change of the scanning rate at the receiver still differs slightly from the scanning rate at the transmitter so as to permit a still finer adjustment in synchronism. When a more substantial degree of synchronism is achieved, the scanning rate at the receiver is changed so as to exactly equal the scanning rate at the transmitter.
The above-described method of synchronism is disclosed in U.S. Pat. No. 4,044,383--Vandling assigned to the assignee of this invention. It is also disclosed in U.S. Pat. No. 3,582,550--Latanzi et al. In both of these patents, the scanning rate at the receiver assumes constant levels which are maintained until such time as the desired degree of synchronization is achieved.
Although the above-described synchronization technique is effective, the technique is time consuming. Generally speaking, there are certain time limitations imposed on achieving synchronization between a transmitter and a receiver. In particular, the CCITT Standards permit approximately six seconds for phasing or achieving synchronization in an AM mode and approximately fifteen seconds for achieving phasing or synchronization in an FM mode. While phasing using the above-described technique can be achieved within this time limitation imposed by the CCITT Standards under the proper circumstances, automatic paper or sheet handling substantially reduces the amount of time available for phasing or synchronizing since paper for sheet handling to the scanning mechanism may, at least in some instances, occur during the phasing or synchronizing period. As a practical matter, even assuming rapid sheet feeding such as that disclosed in copending application Ser. No. 120,461 filed Feb. 11, 1980, this may reduce the amount of time available for paper feeding to less than three seconds in the AM mode and less than seven seconds in the FM mode.